From Ohio History Central

Clams, sometimes called pelycopods (Class Pelecypoda) are filter-feeding animals with two identical shell halves. Some burrowed in bottom mud, whereas others rested on the sea bottom and a few were capable of swimming. Marine species are abundant and diverse in Ordovician through Permian rocks in Ohio and are commonly well preserved. Pennsylvanian and Permian rocks yield species that lived in fresh or brackish waters. Clams are common in some sediments deposited in lakes or streams during the latter part of the Pleistocene Ice Age.

Particularly interesting to fossil collectors are the large clams found in Middle Silurian rocks that are called “beef heart” clams because of their size and resemblance to the heart of a cow. This species, Megalomoidea canadensis (formerly called Megalomas) can reach almost 10 inches in length. They are commonly preserved as internal molds of the shell as the original calcium carbonate of the shell has been leached away. Clams from Mississippian and Pennsylvanian rocks have rarely been found with the original color patterns preserved on the shell. These occurrences remind us that in life many of the fossil organisms we find in Ohio rocks were not dull gray in color as they appear as fossils but had bright color patterns.